Photoconductive thermoplastic lamina

ABSTRACT

This invention relates to a photoconductive lamina having first and second opposite surfaces and comprising thermoplastic material, the softening temperature of the material in the region of the first surface being higher than that of the material in the region of the second surface, and there being at least one intermediate region of material between the two surface regions, the material in the intermediate region having a softening temperature between the softening temperature of the material in the region of the first surface and that of the material in the region of the second surface. The invention also relates to a process for manufacturing the novel lamina.

United States Patent Moraw et al.

[ 1 Oct. 28, 1975 1 PHOTOCONDUCTIVE THERMOPLASTIC LAMINA [75] Inventors: Roland Moraw; Giinther Sch'adlich,

both of Naurod, Wiesbaden, Germany [73] Assignee: Hoechst Aktiengesellschaft, Germany [22] Filed: Dec. 19, 1973 [21] Appl. No.: 426,353

[30] Foreign Application Priority Data Dec. 22, 1972 Germany 2262917 [52] US. Cl. 96/1.5; 96/1.1; 117/218 [58] Field of Search 96/1.1, 1.5; 117/218 [56] References Cited UNITED STATES PATENTS 3,196,008 7/1965 Mihajlov.... 96/1.l 3,276,031 9/1966 Gaynor 96/l.1 3,284,196 11/1966 Mazza 96/1.1 3,317,316 5/1967 Bean et al 96/1.1 3,526,879 9/1970 Gundlach et al. 96/l.1 X 3,615,387 10/1971 Corrsin et al. 96/1.1 3,615,388 10/1971 Gundlach 96/1.5 X 3,698,892 10/1972 Heurtley 96/].1

3,719,483 3/1973 B'ean 96/1.1

FOREIGN PATENTS OR APPLICATIONS 46-8036 l/1971 Japan 96/1.1

OTHER PUBLICATIONS Shattuck, Thermoplastic Deformation With KTA Mode, IBM Technical Disclosure Bulletin, Vol. 12, No. 2, July 1969, p. 261.

Primary ExaminerNorman G. Torchin Assistant Examiner-John R. Miller Attorney, Agent, or Firm.lames E. Bryan [57] ABSTRACT This invention relates to a photoconductive lamina having first and second opposite surfaces and comprising thermoplastic material, the softening temperature of the material in the region of the first surface being higher than that of the material in the region of the second surface, and there being at least one intermediate region of material between the two surface regions, the material in the intermediate region having a softening temperature between the softening temperature of the material in the region of the first surface and that of the material in the region of the second surface. The invention also relates to a process for manufacturing the novel lamina.

14 Claims, No Drawings PHOTOCONDUCTIVE THERMOPLASTIC LAMINA Thy present invention relates to a photoconductive thermoplastic lamina on which there can be produced, by charging and exposure, a charge image which can be transformed by heating into a deformation image, in particular into a phase hologram.

Photo-thermoplastic recording processes and recording material suitable therefor have been previously pro posed. German Patent No. 1,537,134, for example, discloses such a process and a suitable material which comprises an outer thermoplastic photoconductive polymer layer in close contact with a conductive layer, the polymer layer being composed, for example, of a polypropenyl carbazole and an ester gum. From this the possibility of the thermal erasure of the recording is evident. The thermoplastic photoconductive polymer layer is approximately 1.5 to 2pm thick.

A proposal also has been made in German Auslegeschrift No. 1,229,587, that the thermoplastic polymer layer should be in the form of a transparent thermoplastic dielectric layer provided with a photoconduc tive dielectric layer, which in use may be exposed transverse to the surface of the thermoplastic layer and through the thermoplastic layer. In this material the recording is carried out according to the following steps: charging, exposing, charging and heating, wherein separate process steps may overlap chronologically. Furthermore, U.S. Pat. No. 3,615,388, describes forming a thermoplastic photoconductive material in such a manner that there is a nondeformable intermediate layer between a thermoplastic layer and a photoconductive layer.

The disadvantage in the previously proposed materials is that they are not particularly suitable for the reproduction of large local frequencies. It has been shown that for these materials there is a connection between the optimal formation of the deformation images at a certain local frequency and the thickness of the thermoplastic layer. Thus, the local frequency (lines per mm) which can be reproduced optimally by deformation images approximately corresponds to the reciprocal of twice the layer thickness. Thus, for example, with a layer of thickness of below 10,u.m, i.e. between approximately 5 and pm, well formed deformations can be expected only with local frequencies of up to 200 lines/mm. Only when the layers are thinner is the optimally reproduced local frequency range pushed to higher values. Thus a thermoplastic layer with a thickness of 1.5 to 2am is suitable for recording signals at 330 or 250 lines/mm.

The sensitometric characterization of the deformation images on thermoplastic photoconductive recording materials may be carried out by diffraction gratings of different grating distances produced thereon, wherein the image quality, in particular the image brightness is made to correspond to the deformation depth. A detailed description of this sensitometry is given by Aftergut, Gaynor and Wagner in Photo Sci. 'th 9 (1965) 30. In that article, a 500 mesh grating with a line distance of 0.05 mm is produced on a thermoplastic photoconductive film 0.024 mm thick, i.e. the thickness of the thermoplastic film corresponds exactly to half the line distance of the diffraction grating.

A single diffraction grating of this kind corresponds to the hologram of an infinitely remotepoint. With this the suitability of thermoplastic photoconductive recording materials for recording holograms was shown, for the image of each extensive object is composed of dot images. The special interest in thermoplastic photoconductive recording materials for holography arises because (a) because of the alterations in layer thickness, phase holograms of strong intensity can be obtained, (b) these phase-holograms can be obtained, in accordance with the thermal developing process, in a relatively short time, and (c) the phase-holograms may be erased by the influence of heat.

The disadvantage of the previously proposed thermoplastic photoconductive recording materials, in particular when recording holograms, is their limited width of application. Bright diffraction images or bright reconstructions are obtained only by reproducing local frequencies which approximately correspond to the reciprocal of twice the thickness of the thermoplastic layer. In the case of larger or smaller local frequencies, the brightness of the diffraction images is substantially lower. For example, according to Lin and Beauchamp in Appl. Optics, 9 (1970) 2088, the half-width (the width at half of maximum intensity) of the decrease in brightness in the case of an optimumlocal frequency of 1000 lines/mm is approximately 550 lines/mm; that is, below approximately 700 lines/mm or above approximately 1300 lines/mm reconstructions of small intensity only are obtained from holograms on such recording materials.

The present invention provides a photoconductive lamina having first and second opposite surfaces and comprising thermoplastic material, the softening temperature (as hereinafter defined) of the material in the region of the first surface being higher than that of the material in the region of the second surface, and there being at least one intermediate region of material between the two surface regions, the material in the or each intermediate region having a softening temperature (as hereinafter defined) between the softening temperature (as hereinafter defined) of the material in the region of the first surface and that of the material in the region of the second surface. A charge image can be produced on the lamina by charging and exposure, and this charge image can be transformed by heating into a deformation image, in particular a phase hologram. In this specification a charged lamina is slowly heated on a heating plate with temperature measurement until it becomes turbid by an irregular deformation of its surface, the expression softening temperature" is used to mean the temperature at which, or the lower end of the temperature range over which, the material in question softens.

The lamina of the invention preferably comprises a plurality of, advantageously two, thermoplastic photoconductive layers, each of which layers is partially mixed with the adjacent layer(s) in the region of transition from one to the other. The various layers may comprise the same photoconductor or, alternatively, the photoconductors in the different layers may differ from each other in chemical constitution and/or molecular weight. The lamina of the invention may be mounted on a support.

As the softening temperature of the lamina of the invention shows an overall decrease from the first surface to the second surface thereof, the lamina may be said tohave a thermoplastic gradient. In one embodiment of the invention, the softening temperature decreases continuously, i.e. not spasmodically, from the first surface to the second surface.

By the use of the lamina of the invention, both relatively low and relatively high local frequencies can be recorded with one and the same lamina on account of a larger half-width (width at half of maximum intensity), which has the effect that the irradiating angle in the holographic exposure can be extended over a greater range; the lamina has a relatively high lightsensitivity, so that exposure time can be shortened or lower-energy lasers can be used.

In one method that may be used for producing a lamina in accordance with the invention, at least one first photoconductive thermoplastic layer is coated with a second photoconductive thermoplastic layer which, however, softens at a different, lower temperature than the first layer. In this coating, the individually applied layers should not be mixed completely and homogeneously (i.e. there should be only a certain dissolving of the already applied layer but not a dissolving with the solvent of the second layer solution); a more or less pronounced partial mixture in the region of transition between the component layers is necessary. Particularly good results are achieved when the photoconductive thermoplastic layer is composed of two component layers partially mixed in the transition region, the softening temperatures of which differ by between and 30C.

The number of component layers is optional. It has been proved, however, that in view of a practicable embodiment, more than two or even more than three layers are necessary. With more layers, the coating process would be rather difficult and rather timeconsuming. lt has been shown that by applying two or three coats the desired effect may be achieved. A lamina produced from two component layers is therefore preferred.

The photoconductive, thermoplastic lamina of the invention may be applied to various carrier materials. Examples of suitable carrier materials are transparent foils of, for example, a polyester, foils with an electrically conductive layer, and rigid carrier materials with a, preferably transparent, electrically conductive layer. The application of the layers is generally carried out from solution. Because a homogeneous mixture of adjacent layers must be avoided, there are limitations with respect to the choice of layer components and the solvents to be used. Thus, for example the components of the first layer may be readily soluble in solvents such as tetrahydrofuran, dioxane or chlorinated aliphatic substances but insoluble or, only difficultly soluble, in petroleum-type solvents; in this case, petroleumsoluble components may be used for the second layer.

The main components of the photoconductive thermoplastic lamina are the photoconductor and the binder. Because of their good transparency, the preferred photoconductors are organic photoconductors which, to improve the light-sensitivity, are activated by electron acceptors, for example aromatic nitrocompounds. A suitable system is poly-N- vinylcarbazole, to which 2,4,7-trinitrofluorenone has been added. The desired differentation to solubility may be obtained by means of suitable substituents and- /or ranges of molecular weight. In particular, com pounds with alkyl substituents and/or low molecular weights are suitable for a layer to be applied from lower aliphatic hydrocarbons. For example, monomeric N- vinylcarbazole for coating purposes dissolves sufficiently readily in petroleum. in order to achieve an adequate light-sensitivity it is generally necessary. however, to use polymers of a sufficiently low molecular weight range (below approximately 30,000) and to remove the no longer soluble components. Suitable binders for the individual layers are hydrocarbon resins, polystyrols, their copolymers, chlorinated diphenyls or natural resins such as hydrated rosin ester. The photo conductor and the binder must be compatible and film forming in order to produce turbidityfree layers. Flow agents, for example silicone oils, may be used to increase surface smoothness.

The laminae of the invention fulfill to a high extent the requirements made on them in photoconductivity. The light energy to be used for previously proposed photothermoplastic layers is specified for example by Credelle eta1., RCA Review 3 217, (1972) as approximately l60p.W/cm In some cases, the values obtained when using the laminae according to the present invention fall short of this value by a considerable amount. 1n the case, for example, of component layers with photoconductors of different chemical constitution, the requisite light energy was found to be only 13 uW/cm on irradiation with light of 632.8 mm. The use of photoconductors of different chemical constitution is therefore preferred.

The laminae of the invention are also in a suitable manner thermoplastic. For production, the layer with the higher softening range may be first applied from solution to a substrate, and it is then coated with a layer having a lower softening range. In spite of interim drying a partial intermixing takes place in the region of transition.

The softening temperature range suitable from the point of view of the material stress is between approximately 40C and C. Within this temperature ranges is has proved very advantageous for the softening temperatures of the component layers to differ from each other by 10 to 30C. The heat energy to be supplied for the development of a latent charge image into a corresponding deformation image must be applied very carefully so that the temperature range in which the layer in question begins to become soft is achieved directly. If this temperature is exceeded by a few degrees only, the resulting image is erased again.

The thermal erasure of the deformation images and renewed recording is also possible with the lamina of the invention.

The laminae according to the invention are in particular suitable for recording phase holograms. Taking into account the conditions initially mentioned, recordings of low local frequencies of approximately 200 lines/mm up to almost 1000 lines/mm half-width, with maximum diffraction efficiencies of up to 0.18, may be obtained. The diffraction efficiency is a measure of the brightness of the diffraction image and is defined as the ratio of the intensity of the light diffracted in a first arrangement to that of the irradiated light. The diffraction efficiency for thin phase holograms has a maximum of 0.34.

The relationship, described in the literature for previously proposed photothermoplastic materials, between the local frequency which can be optimally reproduced and the layer thickness of the thermoplastic layer, is not found in the case of the materials of the invention. There is only a low tendency that in altogether thinner layers larger local frequencies may be reproduced; This behavior, different from that of previouslyproposed materials, is a direct effect of the thermoplastic gradient.

The following Examples further illustrate the inventron.

EXAMPLE 1 To produce the recording material with a thermoplastic gradient and to produce a comparative material, the following solutions are made up:

Solution A g of a coumarone-indene resin with a softening point of 80C (Gebagan J 80 of VFT, Essen),

10 g of a bromopyrene resin, produced by condensing 3-bromopyrene and paraformaldehyde in glacial acetic acid in the presence of anhydrous zinc chloride,

1 g 9 dicyanomethylene2,7dinitrofluorene, and

120 g tetrahydrofuran with 3 drops of silicone oil. Solution B 100 ml of an aliphatic, branched-chain hydrocarbon, boiling range 160 to 180C (Isopar G, (Esso AG 0.25 g of a mixture of approximately the same parts by weight of condensation products of mono-tbutylpyrene and di-t-butylpyrene with paraformaldehyde, produce in glacial acetic acid in the presence of anhydrous zinc chloride,

0.025 g 9-dicyanomethylene-2,7-dinitrofluorene,

5 g of low molecular weight poly-alpha-methyl styrol (Dow Resin 276 V 9, Dow Chemical Corporation, U.S.A.), and

5 g of chlorinated diphenyl, melting point 60 to 70C (Clophenharz W. Bayer).

Several sheets ofa 50;]. thick polyester foil are coated on a rotating centrifuge with solution A, and then dried at 60C for 30 minutes. Some of the coated sheets are put aside for comparison measurements and the rest are coated on the already coated side with solution B and dried again. In addition, some sheets are coated three times with solution B only, being dried between each coating, in order to obtain a layer sufficiently thick for measurements.

In a preliminary test the thermoplasticity and the photoconductivity of the laminae produced from solution A and those produced from solution B are examined. In addition, coatedfoils are placed on a heatable grounded metal plate. While the temperature is increased, the laminae are charged positively and electrostatically by a corona set up in front of them. When a temperature of approximately 50C is reached, the lamina produced from solution B becomes turbid as a result of irregular surface deformations (frost formation) and when a temperature of about 75C is reached, the lamina produced from solution A likewise becomes turbid.

For the purpose of examining the photoconductivity, the charged laminae A and B are exposed to a rough light-shadow pattern (beam ofa He-Ne laser) and then placed for a few seconds on the metal plate heated to 75C or 50C. Turbidity occurs only in the areas not struck by light.

In the main test, foils with the combined larrLa-from solutions A and B and, for comparison, foils ith the lamina from solution A are fixed with the co on the outside by means of an adhesive strip rier plate of glass 50 X 50 mm, which is pro l with as electrically conductive, grounded layer. The surface resistance of the conductive layer is l9 ohm/square.

Charging is effected by means of a needle corona at a distance of 5 mm, to which is applied a voltage of +8 kv. Irradiation is carried out by the light, divided and brought together again at various predetermined angles, of an He-Ne laser of a total capacity in the exposure plane of 1600 .tW/cm Then a voltage of 20 volts is applied to the leads of the opposite edges of the conductive layer on the carrier plate, for a predetermined developing time, during which time the diffraction grating is formed. The test data for the individual laminae are listed in Table I together with the local frequencies of the maximum diffraction efficiencies and the halfwidth of the local frequencies, at which the diffraction efficiencies are only half of the maximum.

It is clear from this that the half-widths and thus the recording width (lines/mm) are considerably increased.

EXAMPLEZ SolutionA 10 g of a high molecular weight poly-N- vinylcarbazole (Luvican M 170, BASF),

2 g of low molecular weight poly-alpha-methyl styrol (Dow Resin 276 V 9, Dow Chemical Corp., U.S.A.),

250 ml tetrahydrofuran with 1 drop of silicone oil,

and

10 ml of a 15 percent by weight solution of 2,4,7-

trinitrofluorenone in tetrahydrofuran.

Solution B 1 g poly-N-vinyl carbazole of a molecular weight of approximately 20,000,

20 ml tetrahydrofuran,

ml petroleum ether with a boiling range between 80 and 1 10C are added while stirring, and the solution filtered.

The following are also added to the solution:

1 ml of a 15 percent by weight solution of 2,4,7-

trinitrofluorenone in tetrahydrofuran,

4 g of a glyeerine ester of hydrated rosin (Staybelite Ester IO, Hercules Powder, U.S.A.), and 1 drop of silicone oil.

In a manner similar to that described in Example 1, the solutions A and B are applied to transparent polyester foils coated in aluminum by vapor deposition.

The result of the preliminary examination of thermoplasticity and photoconductivity is positive; the soften ing range of the lamina produced from solution A is found to be to C, and the softening range of that produced from solution B is found to be approximately 75C.

The results of the main test are compiled in Table II:

TABLE I1 Lamina (A-l-B) Lamina (A) Exposure energy (uW/cm) 160 160 Developing time (sec) 8.5 l 1 Local frequency (l/mm) of maximum diffraction efficiency 420 550 Maximum diffraction efficiency 0.12 0.04 Half-widths l/mm) 300 to 880 370 to 770 EXAMPLE 3 Solution A 7 g of a high molecular weight poly-N-vinyl carbazole (Luvican M 170, BASF),

1.5 g of a low molecular weight polya1pha-methyl styrol (Dow Resin 276 V 9, Dow Chemical Corp., U.S.A.),

250 ml tetrahydrofuran with 1 drop of silicone oil,

and

7 ml of a percent by weight solution of 2,4,7-

trinitrofluorenone in tetrahydrofuran.

Solution B 0.1 g of a mixture of approximately the same parts by weight of condensation products of mono-t-butyl pyrene and of di-t-butyl pyrene with paraformaldehyde, produced in glacial acetic acid in the presence of anhydrous zinc chloride,

b 4 g ofa glycerine ester of hydrated rosin (Staybelite Ester l0, Hercules Powder, U.S.A.), and

100 ml petroleum ether with a boiling range of between 80 and 100C.

Solution B is shaken for half an hour and then fil tered. Then there is added drop by drop, while stirring, 0.1 g 9-dicyanomethylene-2,7-dinitrofluorene dissolved in 3 ml tetrahydrofuran. Subsequently, the lightgreen solution is filtered, and finally 1 drop of silicone oil is added.

The solutions A and B are applied, as described in Example 1, to transparent polyester foils coated with 7 aluminum by vapor deposition. The thermoplasticity TABLE 111 Lamina (A- l-B) Lamina (A) Exposure energy W/cm") 13 90 Developing time (sec) 80 10 Local frequency (llmm) of maximum diffraction efficiency 580 650 Maximum diffraction cfficiency 0.18 0.08 Half-widths l/mm) 210 to 950 380 to 910 The values for the half-widths indicate the larger recording range (lines/mm) of the materials of the invention.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

l. A lamina composed of at least two thermoplastic, photoconductive layers and having first and second opposite surfaces, the overall softening temperature of the entire lamina being between about 40 and C and the softening temperature of the material in the region of the first surface being about 10 to 30C higher than that of the material in the region of the second surface, and there being at least one intermediate region of material between the two surface regions, the material in the intermediate region having a softening temperature between the softening temperature of the material in the region of the first surface and that of the material in the region of the second surface.

2. A lamina composed of at least two photoconductive, thermoplastic layers on a support, on which there can be produced by charging and exposure a charge image which can be transformed by heating into a deformation image, in particular into a phase hologram, the photoconductive, thermoplastic lamina having a thermoplastic gradient in the direction towards its free surface, which indicates a decrease in the softening temperature of about 10 to 30C, and the overall softening temperature of the lamina being between about 40 and 100C.

3. A lamina as claimed in claim 1 which comprises a plurality of thermoplastic photoconductive layers, each of which layers is partially mixed with the adjacent layer(s) in the region of transition from one to the other.

4. A lamina as claimed in claim 3 wherein the component layers comprise the same photoconductor.

5. A lamina as claimed in claim 3 wherein the component layers comprise photoconductors which differ from each other in chemical constitution.

6. A lamina as claimed in claim 3 wherein the component layers comprise photoconductors which differ from each other in molecular weight.

7. A lamina as claimed in claim 3 wherein there are two layers.

8. A lamina as claimed in claim 1 wherein the softening temperature decreases continuously from the first surface to the second surface.

9. A composite structure which comprises a lamina as claimed in claim 1 and a support therefor.

10. A structure as claimed in claim 9 wherein the support comprises a transparent foil.

11. A structure as claimed in claim 9 wherein the support comprises a rigid carrier material.

12. A structure as claimed in claim 9 wherein the support comprises an electrically conductive material.

13. A lamina according to claim 2 in which the support is electrically conductive.

14. A process for manufacturing a lamina as claimed in claim 1 which comprises applying to a substrate a mixture comprising a first thermoplastic material, a photoconductor and a solvent for the first thermoplastic material, removing the solvent and applying to the layer so formed a mixture comprising a second thermoplastic material having a lower softening temperature than the first thermoplastic material, a photoconductor, and a solvent for the second, but not for the first,

thermoplastic material, and removing the solvent. 

1. A LAMINA COMPOSED OF AT LEAST TWO THEROMOPLASTIC, PHOTOCONDUCTIVE LAYERS AND HAVING FIRST AND SECOND OPPOSITE SURFACES, THE OVERALL SOFING TEMPERATURE OF THE ENTIRE LAMINA BEING BETWEEN ABOUT 40* AND 100*C AND THE SOFTENING TEMPERATURE OF THE MATERIAL IN THE REGION OF THE FIRST SURFACE BEING ABOUT 10* TO 30*C HIGHER THAN THAT MATERIAL IN THE REGION OF THE SECOND SURFACE, AND THERE BEING AT LEAST ONE INTERMEDIATE REGION OF MATERIAL BETWEEN THE TWO SURFACE REIONS, THE MATERIAL IN THE INTERMEDIATE REGION HAVING A SOFTNING TEMPERATURE BETWEEN THE SOFTENING TEMPRATURE OF THE MATERIAL IN THE REGION OF THE FIRST SURFACE AND THAT OF THE MATERIAL IN THE REGIO OF THE SECOND SURFACE.
 2. A lamina composed of at least two photoconductive, thermoplastic layers on a support, on which there can be produced by charging and exposure a charge image which can be transformed by heating into a deformation image, in particular into a phase hologram, the photoconductive, thermoplastic lamina having a thermoplastic gradient in the direction towards its free surface, which indicates a decrease in the softening temperature of about 10* to 30*C, and the overall softening temperature of the lamina being between about 40* and 100*C.
 3. A lamina as claimed in claim 1 which comprises a plurality of thermoplastic photoconductive layers, each of which layers is partially mixed with the adjacent layer(s) in the region of transition from one to the other.
 4. A lamina as claimed in claim 3 wherein the component layers comprise the same photoconductor.
 5. A lamina as claimed in claim 3 wherein the component layers comprise photoconductors which differ from each other in chemical constitution.
 6. A lamina as claimed in claim 3 wherein the component layers comprise photoconductors which differ from each other in molecular weight.
 7. A lamina as claimed in claim 3 wherein there are two layers.
 8. A lamina as claimed in claim 1 wherein the softening temperature decreases continuously from the first surface to the second surface.
 9. A composite structure which comprises a lamina as claimed in claim 1 and a support therefor.
 10. A structure as claimed in claim 9 wherein the support comprises a transparent foil.
 11. A structure as claimed in claim 9 wherein the support comprises a rigid carrier material.
 12. A structure as claimed in claim 9 wherein the support comprises an electrically conductive material.
 13. A lamina according to claim 2 in which the support is electrically conductive.
 14. A process for manufacturing a lamina as claimed in claim 1 which comprises applying to a substrate a mixture comprising a first thermoplastic material, a photoconductor and a solvent for the first thermoplastic material, removing the solvent and applying to the layer so formed a mixture comprising a second thermoplastic material having a lower softening temperature than the first thermoplastic material, a photoconductor, and a solvent for the second, but not for the first, thermoplastic material, and removing the solvent. 